1. Field of the Invention
This invention relates to a process for producing an insoluble tannin which can adsorb actinides such as uranium, thorium, transuranium elements, etc.; heavy metal elements such as cadmium, lead, chromium, mercury and iron, etc.; or metal elements such as cobalt, cesium, strontium, etc.
Also, it relates to a method for adsorbing hexavalent chromium by using the insoluble tannin.
2. Background Art
Nuclear fuel elements such as uranium, thorium, etc. are contained in the waste liquid effluent from nuclear fuel processing facilities. As a process for producing the adsorbing agent for adsorbing the nuclear fuel elements, the prior art discloses a process for producing an insoluble tannin by dissolving a condensed tannin powder in an alkaline solution, mixing the resulting solution with an aqueous aldehyde solution to form a gel composition, and aging the gel composition at room temperature or under heating to stabilize it (U.S. Pat. No. 5,320,664).
Also the prior art discloses a process for producing an insoluble hydrolysable tannin which comprises dissolving a hydrolysable tannin powder in an aqueous ammonia, mixing the resulting solution with an aqueous aldehyde solution to form a precipitate, heating the precipitate, soaking the heated precipitate in a mineral acid such as nitric acid, filtering the resulting mixture to leave a residue on a filter and drying it (U.S. Pat. No. 5,274,169).
The insoluble tannins obtained by the above-mentioned two methods have characteristics that they have a high ability of adsorbing metal elements and low flow resistivity when used in a packed column of the waste liquid effluent treating apparatus. When the water content of the insoluble tannin which is an adsorbent is controlled, the network structure of the gel changes so that its molecular space suitable for the size of the metal element ion to be adsorbed can be retained whereby there is a possibility that an adsorption capacity and adsorption rate of the metal element can be changed. Particularly, the insoluble tannin disclosed in U.S. Pat. No. 5,274,169 has a merit that it is resistant to crush when packed in a column and the adsorption rate of the metal element is more improved by contacting with a mineral acid to harden the tannin surface. Also, it has a merit of preventing occurrence of mold at the insoluble tannin by maintaining the gel to acidic conditions.
However, the ranges of water contents of the insoluble tannins controlled by the above-mentioned two methods are limited only to about 70% to about 85% in practical use.
Also, in the above-mentioned two methods, the insoluble tannins obtained by aging the gel composition at room temperature or under heating for stabilization are crushed into particle size by a mechanical means such as a mixer, etc. and sifted out a desired size so as to be packed in a column, etc. However, the particle size and shape of the insoluble tannin are not constant and when a fine particle insoluble tannin is desired, it must be finely pulverized specifically.